Compound solar cells including a light absorbing layer made of a Group I-III-VI compound semiconductor such as a CuInSe2 (CIS) compound semiconductor containing Group Ib, IIIb and VIb elements or a Cu(In,Ga)Se2 (CIGS) compound semiconductor prepared in the form of solid solution by incorporating Ga into CuInSe2 are known to be advantageous in that they can be produced in the form of a thin film as having a higher light conversion efficiency (“light conversion efficiency” will hereinafter be referred to simply as “conversion efficiency”), and are less liable to suffer from reduction in conversion efficiency due to light irradiation or the like.
A buffer layer of such a CIGS type solar cell including the light absorbing layer made of the CIS or CIGS (hereinafter referred to collectively as “CIGS type”) compound semiconductor is generally formed by chemically depositing CdS, Zn(O,S) and the like (see, for example, PTL1). Where the formation of the buffer layer is achieved by the chemical deposition method, the solar cell is produced by forming the CIGS type compound semiconductor layer in a vacuum by a vapor deposition method or a selenization method, once taking out the resulting product to an atmospheric environment to form the buffer layer, and forming a transparent electrode layer again in a vacuum. Problematically, this reduces the productivity.
To cope with this, it is proposed to form the buffer layer by a sequential sputtering method in a vacuum without the need for taking out the product to the atmospheric environment (see, for example, PTL2).